View Full Version : Boiler Question: Weil Mclain

02-06-2009, 08:34 AM
There is a Weil Mclain boiler (installed in 95) in the house I recently purchased. It's a closed loop system with three zones.

The system has a smaller pipe (3/4 or an inch) that goes between the main incoming and outgoing water lines. The smaller pipe that connects the mains has a lever valve on it (I'm assuming this is the bypass valve). A heating contractor told me the lever should be just partially open to allow some of the water on the hot side to warm up the cooler water of return side before it enters the boiler. The problem is the system is very loud when valve is anything but completely closed or open (causes all sorts of strange surging and clunking noises in the pipes). I had someone else look at the system and he said the valve is usually just partially opened, but since mine was making so much noise it might not hurt to completely open the valve. He didn't seem real sure of himself and now I'm completely confused. Should this connection be completely open or not? If it is left open, doesn't that just bypass the rest of the heating system or, at best, create a lot of inefficeincies? Any help would be greatly appreciated.

My second question is how detrimental it is to run glycol-based antifreeze in a closed-loop system like mine? I'd like to leave the basement zone off, or set very low, but some sections of line at the exterior wall will freeze if I do that (already happened once).

Thanks again for any help. Boiler systems are new to me, and I sure am learning a lot.

02-06-2009, 09:05 AM
It sounds like you may have one of the following conditions: an oversized circulator pump, or you need bypasses on the zones (to relieve the high pressure when say only one zone is open). Do you have more than one circulator pump? Sometimes there will be a circulator for the (each of the) primary loop(s) and another for the secondary loop. Each must be sized appropriately to work well.

Some systems are set up with primary/secondary loops - this ensures optimum transfer and flow through the boiler (if I understand the principle adequately). If set up properly, that throttling valve would normally be open. That cross-connection needs to be within a very close calculated position for it to work properly. Dont' remember the details, but it is something like 'it must be within x to y pipe diameters of z'. It's probably described in the installation manual.

02-06-2009, 09:14 AM
Thanks for the reply.

The system, which feeds my baseboard radiators via three zones, just has one circulating pump. If the bypass valve is supposed to be fully open, that works great for me because that's when the system is its quietest (virtually silent). My concern, however, is that two different contractors have told me the valve usually is just partially open. It also seems to me that having the valve totally open would just bypass the heating system by circulating freshly heated water immediately back into the return. Is that the case or is there something else going on that I'm not understanding?


02-06-2009, 11:15 AM
Not quite...if I understand you, that valve is acting more like a tempering valve to ensure your boiler doesn't get shocked by excessively cold water - it mixes some of the output (hot) water with the return (cold) to ensure it is at least warm.

A pressure relief by-pass valve would be after the circulator and before the zone valves. The circulator is designed to provide good flow to all zones when all are opened. But, that may be too much flow if one or more zones are closed. To keep the flow rates in check, some systems need a bypass valve that automatically opens to relieve extra pressure caused when one or more zones is closed (it normally shouldn't open if all zones are open to ensure you get adequate flow in all of them). This relief valve would bypass hot water back to the return side of the boiler/pump only in enough volume to keep the pump happy while still sending hot to the zone requesting heat.

With your bypass valve partially open, it probably only makes noise when you only have one or two zones calling for heat, but not all three. It's acting like a pressure relief/by-pass, and the restriction is causing problems since the circulator is still trying to pump more water than the less than max number of zones are open.

Later on, when the pros get home from making a living, you may get some more thoughts...

02-07-2009, 05:05 PM
Can anyone help? The link below will show you pictures of my boiler. It's the valve with the yellow handle that I need to know more about -- specifically if it should be open, closed or somewhere in between and why.

Thanks in advance for any responses.


02-07-2009, 06:15 PM
the bypass piping is way too big and located in the wrong place. Just close it and forget about it.